Indomethacin and staurosporine reverse alpha 2 inhibition of water transport in rat IMCD

PKC stimulated by glucagon decreases UT-A1 urea transporter expression in rat IMCD

It is well-known that glucagon increases fractional excretion of urea in rats after a protein intravenous infusion. This effect was investigated by using: (a) in vitro microperfusion technique to measure [(14)C]-urea permeability (Pu x 10(-5)cm/s) in inner medullary collecting ducts (IMCD) from normal rats in the presence of 10(-7)M of glucagon and in the absence of vasopressin and (b) immunoblot techniques to determine urea transporter expression in tubule suspension incubated with the same glucagon concentration. Seven groups of IMCDs (n = 47) were studied. Our results revealed that: (a) glucagon decreased urea reabsorption dose-dependently; (b) the glucagon antagonist des-His(1)-[Glu(9)], blocked the glucagon action but not vasopressin action; (c) the phorbol myristate acetate, decreased urea reabsorption but (d) staurosporin, restored its effect; e) staurosporin decreased glucagon action, and finally, (f) glucagon decreased UT-A1 expression. We can conclude that glucagon reduces UT-A1 expression via a glucagon receptor by stimulating PKC.

Angiotensin II increases vasopressin-stimulated facilitated urea permeability in rat terminal IMCDs

Angiotensin II receptors are present along the rat inner medullary collecting duct (IMCD), although their physiological role is unknown. Because urea is one of the major solutes transported across the terminal IMCD, we measured angiotensin II's effect on urea permeability. In the perfused rat terminal IMCD, angiotensin II had no effect on basal urea permeability but significantly increased vasopressin-stimulated urea permeability by 55%. Angiotensin II, both without and with vasopressin, also increased the amount of (32)P incorporated into urea transporter (UT)-A1 in inner medullary tissue exposed to these hormones ex vivo. Because angiotensin II activates protein kinase C, we tested the effect of staurosporine (SSP). In the absence of angiotensin II, SSP had no effect on vasopressin-stimulated urea permeability in the perfused terminal IMCD. However, SSP completely and reversibly blocked the angiotensin II-mediated increase in vasopressin-stimulated urea permeability. SSP and chelerythrine reduced the angiotensin II-stimulated (32)P incorporation into UT-A1 in inner medullary tissue exposed ex vivo. We conclude that angiotensin II increases vasopressin-stimulated facilitated urea permeability and (32)P incorporation into the 97- and 117-kDa UT-A1 proteins via a protein kinase C-mediated signaling pathway. These data suggest that angiotensin II augments vasopressin-stimulated facilitated urea transport in the rat terminal IMCD and may play a physiological role in the urinary concentrating mechanism by augmenting the maximal response to vasopressin.

Regulation of cAMP production in initial and terminal inner medullary collecting ducts

BACKGROUND

The inner medullary collecting duct (IMCD) is composed of at least two functionally and morphologically distinct segments, the initial (IMCDi) and the terminal (IMCDt) portions. However, most studies of receptor signaling have been performed on cells obtained from the entire inner medulla. The purpose of this study was to determine whether the patterns of receptor-activated cAMP accumulation were different between these segments.

METHODS

We measured cAMP accumulation stimulated by vasopressin and isoproterenol, and the effect of epinephrine in freshly dissected IMCDi and IMCDt segments cultured and IMCDi and IMCDt cells in primary culture.

RESULTS

The maximum response to vasopressin was twofold higher in fresh IMCDt verus IMCDi (P < 0.05), however, it increased in cultured IMCDi by 40% verus fresh cells with no change in the response in fresh verus cultured IMCDt. The maximum response to isoproterenol was small in fresh cells but increased by five- and sixfold, respectively, in cultured IMCDi and IMCDt cells. alpha 2-Adrenoceptor stimulation almost completely inhibited both vasopressin and isoproterenol-stimulated cAMP accumulations in fresh IMCDi and IMCDt cells, but only partially inhibited either accumulation by 34 to 49% in cultured cells.

CONCLUSIONS

(1) IMCDi and IMCDt cells are both subject to vasopressin and alpha 2- and beta-adrenergic regulation of adenylyl cyclase activity; (2) the relative influence of beta-adrenergic, alpha 2-adrenergic and V2 receptors to affect cAMP accumulation is altered in primary culture versus freshly dissected IMCD segments, suggesting that caution must be exercised in the extrapolation of data from cultured IMCD cells to in vivo models.